In general, radiography systems for medical diagnosis irradiate X-rays onto a human body and detect an energy intensity distribution difference between the X-rays transmitted by the human body. Detection methods of these radiography systems are divided into an analog type and a digital type.
In the analog detection method, an intensifying screen (a fluorescent plate) which emits light when it receives X-rays and a silver salt film are combined, a latent image is formed on the silver salt film using the light generated by the intensifying screen, and then chemical treatment is carried out on the silver salt film, thereby obtaining a visible image. In order to perform such a method, the intensifying screen and the silver salt film are kept in a dark condition, and a film cassette is used in radiography.
In the above analog detection method, consumable costs of the film are continuously generated during a development process, the film is used to perform all processes, such as image acquisition, display, storage, and transmission media, and thus requires close attention and causes a difficulty in maintenance of the film, increase of maintenance costs, and a lot of risk of loss and damage, a development waste liquid causes environmental pollution and thus obstructs hospital environments, and time consumption due to the development process delays a consultation time of a patient and thus improvement in medical service environments is urgently required.
In the digital detection method, a two-dimensional sensor is provided as a detection medium responding to X-rays, a minute electrical signal generated by the sensor is obtained through a two-dimensional matrix and is amplified by an amplification circuit, the signal amplified by the amplification circuit is converted into a digital value by an analog/digital converter, the digital value is changed into an image data, and the image data undergoes proper image pre-processing for optimal visualization and is then displayed on a monitor or a printer.
Representative detectors employing the above method include Computer Radiography (CR) using image plates, a CCD detector using a scintillator and a Charge-Coupled Device (CCD), a flat panel detector using a thin film transistor, and so on. Among these detectors, a digital X-ray image obtaining method using the flat panel detector has been increasingly used now in consideration of image quality, reduction in an amount of radiation exposed to a patient, efficiency in inspection, and easiness in mount.
An ideal flat panel detector must have high contrast resolution and high spatial resolution at a proper amount of radiation so as to provide an image of a high quality. Further, the ideal flat panel detector must have high time resolution, achieve imaging at a large area of more than 14×17 inches, and have a matrix having 2000 pixels in the horizontal axis direction and 2000 pixels in the vertical axis direction with a pixel size of less than 200 μm. Moreover, the detector must have a strong frame, use semiconductor, and have a size equal to that of an intensifying screen/film cassette and proper weight and performance.
Conventional flat panel detectors used for the clinical purpose mostly satisfy the above requirements, but are not satisfactory in size and weight and are used only in a state in which the flat panel detectors are fixed to the inside of a table bucky or a stand bucky, thus causing several problems, as follows.
First, special radiography techniques for diagnosis are limited. For example, the fixed flat panel detectors are fixed to the inside of the closed table bucky and the closed stand bucky, X-rays generated from an X-ray tube head are irradiated downwards and are transmitted by a patient, and the fixed flat panel detector fixed to the inside of the closed table bucky detects the transmitted X-rays. However, since movement of the fixed flat panel detector is restricted, radiography in the horizontal direction or radiography in the axial direction is not performed or radiography is performed only in an inconvenient pose of a patient, and thus acquisition of a diagnostically valuable image is limited. Particularly, if a serious patient or an emergent patient needs to be radiographed as being located on a portable patient bed, the digital radiography system using the fixed flat panel detectors cannot perform radiography of such a patient.
In order to solve such a problem, the conventional digital radiography system using the fixed flat panel detectors must be used together with Computer Radiography (CR) equipment using image plates.
The image plate is used under the condition that it is mounted on a protective case referred to as a cassette and having the same shape and weight as those of a film cassette using the intensifying screen and the film, thereby providing familiarity to users having used the film cassette. Further, the image plate has a weight of about 1˜3 kg, and thus may be applied to various radiography postures using a simple fixing device. The image plate has a latent image generated by radiography, and the latent image is read by an image reader and is converted into a final image through analog/digital conversion and image processing. However, the image plate is deteriorated according to the number of times of radiography, and needs to be replaced with a new one through image deterioration measurement.
If both radiography using the flat panel detector and radiography using the image plate are performed with respect to one patient, diagnosis through comparison between two digital images is very difficult. Since the two detectors are different in aspects of characteristics and image processing techniques thereof, it is difficult for an image reading doctor to diagnose the patient through comparison between the two digital images.
Second, a grid to remove scattered radiation generated from a subject for radiography due to X-ray irradiation is fixed to the bucky. The grid is used even in radiography which generates a small amount of scattered radiation and thus does not require the grid.
The grid serving to remove the scattered radiation reaching an X-ray detection medium includes a lead foil plate and aluminum (or paper or carbon). The grid removes primary rays required to form a precise image as well as removes the scattered radiation. If the grid is fixed to the bucky, the grid must be used even in radiography not requiring the grid, and thus radiography conditions are raised as much as a sufficient amount of primary rays due to use of the grid, thereby causing increase in an amount of radiation exposed to a patient and raise in load of an X-ray tube. In order to improve these problems, some equipments are provided with buckies designed such that grids may be detachably attached to the buckies.
Third, the fixed flat panel detector is fixed to the inside of the bucky, and thus all radiography images are enlarged (iPs/Ps, Ps: size of subject for radiography, iPs: image size).
As a radiography image for medical diagnosis, an image having the precisely same size as that of a human body (Ps=iPs) without enlargement is required. In order to minimize image enlargement, a distance Dfo between an X-ray focus and a patient is maximally elongated and a distance Dod between the patient and the detector is minimized. In radiography on a table, since a distance D fod between the X-ray focus and the detector is defined as 100 cm, in order to minimize image enlargement, the distance Dod between the patient and the detector is minimized, but the fixed flat panel detector is mounted within the bucky and thus minimization of the distance Dod is limited.
Fourth, if the detector is replaced, it takes a long time to complete the replacement until service complement.
If the fixed flat panel detector is out of order during using the equipment and cannot be used, the digital radiography system cannot be used, and a time for service engineer call, a time for detachment of the flat panel detector and mount of a new flat panel detector and for detector stabilization, and a time for image correction file generation of the detector (for calibration) are required. Therefore, the expensive digital radiography system cannot be used for a long period of time.
Recently, in order to solve the problems of the digital radiography system using the fixed flat panel detectors, a digital radiography system using portable flat panel detectors (hereinafter, referred to as ‘electronic cassettes’) is being used. The electronic cassettes must be small-sized and light-weight in consideration of mobility and operability, and particularly, in order to apply the electronic cassettes to the conventional radiography systems using image plate cassettes or film cassettes, a specific structure of the electronic cassettes needs to be designed.
Particularly, the electronic cassette selects a wireless data transmission method in order to assure autonomy thereof, requires a storage battery for power supply, and requires a special structure to minimize damage due to external impact according to autonomy and a special design to be interchanged with the conventional image plate cassette or film cassette in the bucky of the radiography system. Thereby, the size and weight of the electronic cassette are increased, and particularly, if the number of times of radiography is increased, the storage battery needs to be designed so as to have a high capacity and thus special configuration and structure are required so as to satisfy miniaturization and light-weight requirements of the electronic cassette.
Further, in case of the conventional image plate cassette or film cassette, when radiography is performed under the condition that the conventional image plate cassette or film cassette is mounted within the bucky, a latent image, which is recorded on the image plate of the image plate cassette and the film of the film cassette, is converted into a final image by the image plate reader or a film developing unit after the cassette is detached from the bucky. Therefore, the image plate cassette and the film cassette repeat mount within the bucky and detachment from the bucky whenever radiography is performed, thereby providing user's inconvenience.